Fix

Author: dsaxton

pandas/core/dtypes/missing.py

@@ -235,7 +235,7 @@ def _isna_ndarraylike_old(obj):
 
     if is_extension_array_dtype(dtype):
         result = values.isna() | (values == -np.inf) | (values == np.inf)
-    if is_string_dtype(dtype):
+    elif is_string_dtype(dtype):
         result = _isna_string_dtype(values, dtype, old=True)
     elif needs_i8_conversion(dtype):
         # this is the NaT pattern